Multi-GHz bandwidth signal processing systems (e.g. radar return processors) commonly make use of acousto optic signal processing devices, such as surface acoustic wave (SAW) devices to spatially segregate signals in terms of frequency. Because of its dispersion characteristics and bandwidth, in order to provide an optimal chirp output signal, the SAW device should be driven with a signal that represents, as closely as possible, an impulse function centered at the resonant frequency of the SAW device. In terms of a practical drive signal, such a signal consists of an prescribed number of (RF) carrier cycles at the resonant frequency of the SAW device.
Because of the behavior of the SAW device, the characteristics of the impulse wave must be precisely defined, in order to minimize carrier leakage and consequential Fresnel distortion. As an illustration, for a center frequency 500 MHz, a SAW dispersion time of 3 microseconds and a bandwidth of 150 MHz, the optimum drive to the SAW device consists of exactly four complete cycles of the 500 MHz carrier with the first order sidelobe nulls spaced 150 MHz apart. The sidelobes are expanded in time by the SAW device over its 3 microsecond dispersion time. Any leakage of the carrier is integrated by the device and appears in the output as a constant frequency (tone), which interferes with the normally dispersed waveform and creates the unwanted Fresnel distortion.
The degree to which the carrier is extinguished in order to effectively suppress this distortion is proportional to the ratio of the carrier cycle time; in terms of the above parameters, 8 nanoseconds divided by the 3 microsecond period, corresponding to 60 dBc for a S/N of 1.0, which is far below typical S/N requirements of 30 dB, requiring an extinction ratio of at least 80 dBc.
Previous attempts to achieve high extinction ratios have included the individual use of mixers, analog switches, and impulse generators. A mixer, to the respective ports of which the RF signal and a pulse signal are applied, can support the requisite rise time, but cannot achieve an extinction ratio in excess of 30 dB. An analog switch, on the other hand, is capable of providing 70 dB of isolation, but is too slow to achieve the requisite rise time. Finally, an impulse generator spreads out the energy from DC to infinity in the frequency domain.